Integrated circuit packages are presently in wide use in the electronics industry in various types of electronic equipment. It has been found that a bad integrated circuit package will normally fail within about one year of the time of its initial use. The failure of an integrated circuit package in actual use can be vary expensive.
To eliminate such failures of integrated circuit packages in use, it has become a common practice to test the integrated circuits by simulating operation for up to a year or more. This testing process is known as a "burn-in" process.
The burn-in process is accomplished by means of a burn-in board. Burn-in boards are printed circuit boards on which a number of sockets which receive an integrated circuit package are soldered. A typical burn-in board is 8 inches wide and 24 inches long and has about 200 individual sockets. Each socket has a plurality of electrical contacts which engage the leads of the integrated circuit package. Each socket contact in turn is electrically connected by the printed circuit to an edge connector on one end of the burn-in board.
Individual integrated circuits are placed in the sockets with each pin of the integrated circuit package making electrical contact through the socket to the edge connector. Once the burn-in board is loaded, it is placed into an oven where the devices are electrically exercised while at an elevated temperature for a period from about 24 to about 96 hours. At the completion of the burn-in process, the boards are unloaded and reloaded with the next batch of integrated circuit packages and the burn-in process is repeated.
Integrated circuit packages are becoming progressively smaller with the leads being placed closer together. The new generations of integrated circuit packages are referred to as surface mounted devices (SMD). SMDs include small outline (SO) integrated circuits, plastic leadless chip carrier (PLCC) and leadless chip carrier, usually ceramic (LCC). The lead spacing of SMDs is typically 0.050 inch center-to-center and may eventually become as small as 0.025 inch center-to-center spacing.
Recently, specialized sockets for receiving SMDs and automatic loaders and unloaders for burn-in boards with the new SMD sockets have been developed. One such socket, a zero insertion force (ZIF) socket, has a lid which is depressed to spread the contacts to facilitate insertion and removal.
The automatic loaders and unloaders typically comprise a series of insertion-extraction heads, each head having a crown and a movable plunger extending through the crown. The SMDs are singulated onto the crown and held in place on the end of the plunger, e.g. by vacuum. Each head is then moved toward a socket until the crown engages the socket and depresses the socket lid. The plunger then moves forwardly, placing the SMD into the socket. While the plunger holds the SMD in the socket, the crown is moved away from the socket releasing the socket lid and securing the SMD in the socket. The plunger is then backed away into its original position ready to insert a new SMD into a new socket. The burn-in board is then advanced and the process repeated.
Removal of SMDs from the sockets generally involves the same series of steps but in reverse order. That is, the extraction head is brought into a position with the crown engaging the socket, but not depressing the lid. The plunger is then moved forwardly to engage the SMD within the socket. The crown is then moved forwardly to depress the lid of the socket and the plunger then backed away to remove the SMD from the socket. The crown is then backed away from the socket and the SMD released directly into a packaging tube.
The close lead spacing of the SMDs requires exact placement of devices into the burn-in sockets. Typical device insertion requirements are .+-.0.002 inch in each of the X, Y, and Z directions referenced to the particular socket being loaded. These insertion requirements tend to be more precise than the socket positions on the burn-in board due to inaccuracies in the placement of sockets on the burn-in boards as well as board warpage caused by the temperature changes during the burn-in process. Accordingly, the crown and plunger of the insertion-extraction heads must be automatically alignable to the exact location and orientation of the socket on the burn-in board, i.e., the crown and plunger must be automatically adjusted in the X and Y directions. In addition, because most SMD sockets have a lid which must be fully depressed to open its electrical contacts for insertion and removal of the SMD devices, the crown and plunger must be automatically adjustable in the Z direction.
Insertion-extraction heads are known wherein the crown is mounted on the main body of the head by a combination of coil and leaf springs. The coil springs are positioned between the main body and the crown. The leaf springs are fixedly attached at their lower ends to the side walls of the head main body and extend upwardly across the side wall of the crown. At their upper ends, the leaf springs have a hole or slot through which a pin, which protrudes laterally outwardly from the side walls of the crown, extends. The slot is larger than the pin and affords the leaf springs and hence the crown movement relative to the main body. The leaf springs generate a "return-to-center" force on the crown.
The above design has several drawbacks. Because the crown is only loosely secured to the head, it is difficult to maintain control of the crown position and orientation. As a result the crown has a tendency to become tilted, i.e. not parallel, relative to the burn-in board and socket. During insertion, such tilting prevents accurate placement of the SMD in the socket and may result in damage to the SMD, socket or even the head. Moreover, the presence of the leaf springs and laterally extending pins means that the heads cannot be mounted in the loader-unloader in close proximity to each other. As a result of the required head spacing, adjacent sockets of a row cannot be loaded or unloaded at the same time. Two passes of the burn-in board are required to load a row of sockets.